There is another interview with Monica Seles I could imagine having written. That's the one in which, with the French Open playing on the TV in the background in her hotel room in Florida, we talked about the fact that she was the greatest female tennis player ever to pick up a racket; about the 20 grand slam titles she won before she bowed out of the game, eclipsing Martina Navratilova's record. The one in which she described how she finally mastered Wimbledon, and could look back on her dominating rivalry with Steffi Graf who, beaten by her nemesis, never quite fulfilled her early promise. But that is not this interview ...

This one dwells on the way that the life that Seles seemed to have ready and waiting for her - eight grand slam victories in her teens - ended violently in April 1993 when she was 19 and a deranged Graf fan ran on to the court at a tournament in Hamburg and stabbed her in the back with a nine-inch kitchen knife, changing her script for ever.

We are in Florida, and the French Open semi-final is playing in the background, but our talk is not of titles won and lost, of epic victories and narrow defeats - it is of the psychological trauma of that defining violent event, and of the decade of disappointment and despair that followed. A decade in which Seles looked everywhere for comfort, "always searching for the key to getting my old life back", and found that comfort primarily in food, an obsession which brought with it many more problems.

Seles is 35, taller than you'd imagine from watching her on court, and much slimmer than in her later playing days. Her voice is still inflected with the giggly girlishness of the tennis prodigy, which makes what she has to say all the more poignant. She drinks black coffee and buzzes determinedly between subjects, just as she once used to chase down every lost cause on court. She has been retired now for five years; she lives alone in Tampa Bay with her four dogs, and she resolutely refuses to deal in "what ifs?" - "I would have gone crazy a long while ago," she says, "if I had done that." She would rather dwell on what she sees as the greatest victory of her life, the one she savours above all others - her triumph over her destructive eating habits and her weight, which is shorthand for her triumph over all of her demons.

She has written a book detailing that long campaign, Getting a Grip. It is a self-help manual and a sports autobiography, a "misery memoir" and the best kind of diet book (one that does not tell you what to eat, but how to live). From the perspective of her retirement Seles unravels all the extremes of her career, extremes that led her close to insanity. At the heart of it is a tale of lost innocence. What once seemed so natural to Seles - her life, her game - became, after the violence that interrupted her, something that she felt she had to make up as she went along.

"I knew I was a tennis player," she writes, by way of introduction, "I knew I used to dominate the sport, and I knew I used to be a happy person, but for 10 years those identities eluded me." She hopes and believes that the ways in which she put her self back together will have a universal application - and she proves the point as soon as she sits down by reading quickly from an emotional email she has just received from a young woman in Italy, a doctor who has been fighting all her life with an eating disorder after a childhood trauma. Seles has been the doctor's inspiration. "I'm always a bit wary of getting involved in fan letters," Seles says, "but this one I will."

The lives of all professional tennis players are about focus, a narrowing down of the field of vision to a simple moving target that must be hit, and lines that must not be crossed. Invariably that focus begins very early (Andre Agassi's father hung a tennis ball above his baby son's cot and let him bat it around all day to improve his hand-eye coordination). Monica Seles was once the most focused five-year-old anyone had ever seen. Her story began, as nearly all tennis stories begin, with her watching her father. One morning on a family holiday on the Adriatic, Seles observed her father and her brother carefully packing a bag with tennis rackets. When she asked where they were going, her brother Zoltan answered: "To play tennis." Seles recalls, she says, hearing only the word "play" from that sentence. It sounded like fun. Could she come and play too?

She never, for many years after that moment, really stopped playing, though it quickly ceased to be anything resembling fun. The Seles family lived in Novi Sad, in Serbian Yugoslavia. Monica's father was a political cartoonist for various newspapers, but in his youth he had been a top athlete, a nationally ranked triple jumper who used to compete barefoot. He regretted that he had not been able to pursue his athletic career and was determined that his children should not have the same regrets. By the time Monica started playing, her brother Zoltan was the top-ranked junior in the country and competing with the young Boris Becker and Stefan Edberg in European events. She quickly developed the ambition of beating him, though he was eight years her senior.

Her father, she says, did not push her, but he did not discourage her either. There were arguments at home - her grandmother and mother would say that it was not natural for a girl to play so much tennis, not ladylike - but neither her father nor Monica would listen. "My dad," she says, "as an artist, was aware of the dangers of too much structure; in particular he was very keen that I should not lose my childish imagination when I was playing." Practice was built around make-believe. Monica was a great lover of TV cartoons, so her father would draw the face of Jerry the mouse on every tennis ball and Monica would be Tom, trying to whack him with her racket as he escaped. She would do this for many hours at a time. They lived in a flat, and children were not allowed at the local tennis club - even children as gifted as Seles - so her father strung a net between two cars in the car park next to their block for Monica to play there, hitting balls into boxes at the court's corners. Sometimes her father would break off from his drawing board and shout down from their third-floor window to ask how she was doing. A hundred or 200 accurate balls into boxes, and she would come in for her supper.

Seles looks back on this as a golden time. The only fears in her life were those that attended losing. I've talked to a few tennis champions over the years - McEnroe, Borg, Agassi, Federer - and though immensely different in character, they were united by one thing: an overwhelming fear of the pain of defeat. It was always that, more than any desire for glory, that drove them on when they were young. Seles, too, was full of that feeling. She recently came across a photograph of herself, she says, aged seven. She had come third in a tournament for girls much older than her, but her face was set in a mask of pure self-loathing. She could not bear it.

By the time she was 13, Seles was the top-ranked under-18 player in the world. She had been spotted the year before at a tournament in the States by the legendary coach Nick Bolletieri and invited to join his academy in Florida. She moved originally with her brother, and later the whole family joined her. Before she went, she knew nothing of the world of tennis. The only match that was shown on TV in Yugoslavia was the French Open final - "Even at 11," she says, "I had the feeling that the only two tennis players in the world were Chris Evert and Martina Navratilova, and they played this one match against each other every year." Her father had encouraged her to just play every point as though it was her last, without thinking of anything else. She was, she says, ignorant of the scoring system in tennis long after she arrived at Bolletieri's school. Even among the single-minded generation there, she was something of a phenomenon. The future world number one, Jim Courier, refused to hit balls with her after one occasion when she had him chasing around the court's four corners in the afternoon sun.

She recalls it now, though, as the beginning of some of her insecurity. "I left my parents and all my friends at 13. It's an age when you are very unsure of your body and everything. I was allowed to call home once a month. I thought I spoke English but when I got to America I realised I didn't really. I had, like, 20 words. I was on scholarship. The other girls could afford to pay to be there, had everything, but I was the only female that was really good. I was very shy. And at the end of the day you are a kid."

Bolletieri spoke of her at the time as the brightest prospect he had ever seen. "She will not accept that she can't do something," he said, "and she'll spend 40, 50, 70 hours working just to get one shot. I used to tell her: 'Your boyfriend is your Prince ball machine', she spent so much time with the thing. You can't yell at her, and she's stubborn; you have to do a lot of proving if she doesn't agree with you. But I find it very difficult to pick out any weakness in her or her game."

Her weaknesses were perhaps, however, beginning to show off court. At the same time as Bolletieri was singing her praises, Seles was suggesting to the New York Times: "As long as I love it, I'll keep playing. Plus I'm still making straight As at school, as always. So now I just worry about my cholesterol. I don't like salads: I like the strong food."

Monica Seles. Photograph: Seth Sabal

None of this anxiety showed on court at the time, however. Seles says she never really thought of herself as having the capacity to be a great player until she beat Steffi Graf in the final of the French Open in 1990 when she was 16 (Graf was five years her senior). After that initial victory over Graf she hardly looked back. I remember watching her then; it was like seeing someone who had rethought the rules of women's tennis; she was so aggressive in her play, and so enclosed in her concentration, it seemed like nothing could get in her way.

For three years, little did. She won practically everything (except Wimbledon), but then the moment came that changed everything. In 1993, she had a realistic chance of winning all four grand slams. She was the Australian champion, and Paris was on the horizon. But as she was sitting with her back to the crowd at a changeover between games at a tournament in Hamburg, Gunther Parche, a 38-year-old who had stalked Steffi Graf for years and hated the fact that Seles had "stolen" the German's number one ranking, changed tennis history by attacking her with a knife.

Seles can talk about the stabbing now, but she does not like to dwell on it too much "because it takes me back to a very dark place in my life". The shock was one thing to cope with, and the physical damage to her shoulder was another - a centimetre to the left and she would have been paralysed for life. But really, she says, none of that was the worst of it: the hardest thing to cope with was the fact that the life she had put all her faith in had disappeared in an instant.

Looking back, Seles suggests, her peak years in tennis would likely have been between the ages of 19 and 22. As it turned out, she hardly picked up a racket at all in that time. The nightmare of her assault deepened almost immediately when she discovered as she lay in hospital that her beloved father had been diagnosed with terminal stomach cancer - he had missed the tournament in Hamburg in order to have tests. For her, the news and the timing could not have been worse. Her dad was her mentor and her best friend; it was to him that she would have turned to express her deepest fears about the horror of what she had experienced. But, she says, "I didn't want to pile other worries on him - he had enough to think about."

Lying in her hospital bed Seles also received a very brutal lesson about the world of tennis. "In terms of the game itself," she says, "it was like the stabbing never occurred. One problem was that it happened in Germany and was 'because' of a German player. The German federation decided to continue the tournament as if nothing had happened, and everyone else seemed to follow on from that."

Graf came to see Seles in hospital "for a minute or two" but there wasn't much to say: they had always been rivals rather than friends. "It was one of those things," she says now, "but it felt like everyone benefited from the stabbing except me." The players were asked to vote on whether, in respect of the unique circumstances, Seles's number one ranking should be retained until more was known about her condition. They voted unanimously against that idea (with one abstention: Gabriela Sabatini), and so everyone moved up a notch and the waters closed over the head of the recuperating champion. "They just wanted me to go away, it felt like," she says. "I was 19 years old. Their money was tied up to the ranking system, and that was obviously an issue..."

Gunther Parche also haunted Seles. He eventually stood trial on a charge of wounding rather than attempted murder, and though he admitted the attack had been premeditated he escaped a prison sentence after psychological reports. "The trial kept going on and on," Seles recalls. "One trial after another. Later I tried to sue the German Tennis Federation for lack of security and lost income, and I lost those cases, too. It was hard to cope with the fact that the guy was not even sent to prison. It did not feel like justice to me."

Every time Seles tried to walk on a tennis court, as her injuries healed, she found she couldn't face it and turned around. "I had grown up on a tennis court - it was where I felt most safe, most secure - and that day in Hamburg everything was taken away from me. My innocence. My rankings, all my income, endorsements - they were all cancelled. And the one person who could have comforted me really, who understood what it meant, my father, was of course facing this awful illness."

Seles started eating. She had always enjoyed her food, never had to be told to clear her plate as a child, and now she did that, and more so. "And of course a plate of food in Florida is bigger than one in Europe." After seeing her father go through chemotherapy and be unable to eat, after putting herself through Olympian fitness regimes in order to get back to playing, she would return at night to the fridge. "Potato chips were my downfall," she says now, with a smile. "Just as I had been a champion tennis player, now I became a champion potato-chip eater." On her 21st birthday, when she might have had the world at her feet, she stayed at home with a bag of cookies, and cried.

"The thing was," she says, "when I thought of coming back I had no idea how I would feel sitting back down on the chair, knowing the person who had stabbed me had never been put in jail. There were so many ifs. In the end though, after two and a half years, I felt I just had to try. I came back in Toronto and the fans' support was just amazing. I won that first tournament back, and that helped. It was like: 'I am still pretty good at this.'"

In some ways though, her problems were only starting. She had worked hard to get into shape for that tournament, but even then she was nothing like the weight she had been at 19. It was then she started to hear the voices.

"I remember coming back to play Martina in an exhibition before the Toronto event and I was maybe 25lb heavier than I had been," she recalls. "And I could hear the comments: 'Oh my God! What happened to Seles? Did you see how big she was?' I mean, I had been nearly stabbed to death. I had been out of the game for two years. My father was extremely sick. I was no longer a teenager. I turned to food for comfort. What did they expect?"

In some ways Seles was prepared for the scrutiny. She had suffered some of it before the stabbing, particularly on one occasion when she had cut her hair into a new style as part of an endorsement deal for a haircare company.

"I went to my first tournament with this new hair and this woman comes up to me. I'd never met her and she said: 'What happened to you - you look like a boy, you look terrible!'"

The new hair had coincided with the controversy surrounding her "grunting" as she hit the ball on court. "Suddenly I was this aggressive boy grunting away." Seles says she was never really aware of her grunting before the media picked up on it, though she had done it since she was a child. Things had come to a head at Wimbledon in 1992, when the papers made a controversy about the noise she made and the players started to complain - notably Martina Navratilova, who lost to Seles in the semi-final.

"I had grunted against those players countless times," she says now. "Nobody ever told me to do it or not to do it. But going into that tournament I had lost one match all year. I think it was a purely a mental tactic, by Martina and others. You always look for something. With me I didn't have a crazy father, I didn't have a crazy personal life, there was just this grunting, so they went for that."

Seles believes the controversy got to her. "It was on my mind a little in the final and I lost to Graf. I grew up a lot that day. And I decided never again would I listen to what people say. If they made grunting against the rules, then I would have to think about it, but otherwise I would do whatever helped me to play my best."

Some of those doubts went through Seles's mind again when she heard people commenting about her weight on her return to the game, but she tried to banish them. It was not easy. "My generation was the last when you were marketed really as a tennis player - Graf, Hingis. But when Anna Kournikova came along, there was this whole other thing - suddenly it was all about looks. Tennis is pretty unforgiving if you are carrying weight. You are expected to wear short skirts, and you are compared to all these 16- and 17-year-olds. Nobody needed to tell me - I only had to look in the mirror or try on my clothes. I tried so hard to lose weight. Every year began with a resolution - I would wake up in the morning thinking about my size, and go to bed at night staring at the ceiling, hungry. I tried this fad diet or that and I lost the weight and then two months later I would gain it back again and more."

Seles won one more major title, the Australian Open in 1996, but though she still wanted to win as much as ever, she could not stop eating long enough to allow her to do so.

Wimbledon was always the lowest point of her year, she says. "I would have played the week before at Eastbourne, where it always rained every day, so there was nothing to do but watch the rain and eat. There was the pressure of playing on grass, which was not my favourite surface, and worse, the British press, which would always be on to me, first about my grunting then about my size." The stress made her compulsion worse.

"There would be pictures of 'Monica's spare tyre' - that would be the headline. I dreaded those fortnights. My heaviest ever was 1997 Wimbledon: my father was very sick, the outfit I had to wear that year didn't help, I was 35lb overweight. You cannot carry that around a grass court. I was reading the articles before I went on court. And then if a player hit a drop shot or something I'd be thinking: 'If I was skinnier I'd have got that ball' and 'Did she do that because of my size?'"

The cycle of seeing her picture in the papers and being alone with room service and a mini-bar did not help. "The British press was so unbelievably cruel. And then at press conferences I would have to sit there while these guys who had written about how fat I was asked me questions. And you know sports writers are not necessarily in the best shape themselves. These enormous guys, asking me if I could be in better shape - I mean, look at yourself in the mirror! Don't be so brutal!"

Seles can laugh about it now, but at the time it was never a joke. She found it hard anyway to form relationships as a tennis player always on the road, but her problems with eating made it all the harder. She recounts some horrific tales in the book of romances that went wrong when her boyfriends took it upon themselves to comment on her size. One, in league with her fitness trainer, promised he would take her out for dinner if she won the Italian Open. She followed her diet all week on that promise, won the tournament, but then her date still voiced his disapproval as she tucked into her tiramisu. Another boyfriend had a habit of pinching at the spare flesh on her midriff and suggesting she needed to watch it.

"A guy would always end up mentioning my weight in some form or other," she says. "They knew they should not go there; it was too painful for me. But they always did. It seemed so simple for them: stop eating, win grand slams, be happy." But Seles knew it wasn't so simple, and that it wasn't just about food.

The question I've been wanting to ask her all through our conversation is whether she believes she would have encountered these problems had it not been for the stabbing. Does she think there was something in her obsessional focus as a young girl that would always have found an outlet in this kind of neurosis? After all, other comparable prodigies - Jennifer Capriati, Martina Hingis- had their share of angst.

She is not sure. "These days I am a great believer in keeping things in balance," she says. "I was paying for that imbalance in my childhood maybe, who knows? Women I have talked to who have a similar problem with food say it is all about control. For me it was the opposite. Food was the one area of my life that was out of control. Everything else was looked after for me. How I did my workouts, what time I went to bed, everything. I had this mental strength on court, but off it I could not win."

Seles kept most of this to herself. She never talked much about how she felt after the stabbing, or about her grief for her father who died in 1998, or about the life she had lost. Instead of therapists she turned to fitness gurus. One month she would have Carl Lewis's trainer, the next Oprah Winfrey's. But the harder she trained the harder she ate: "seven-hour workouts would be followed by 5,000-calorie binges". In the end, as her book details, she had to find out the answer for herself.

It was a series of injuries that started her off - problems with the straining of feet and ankles that eventually brought a premature end to her career. In a period of enforced rest she took a holiday, to "celebrate" turning 30. To start with she read (again) every nutritional book on her shelf - injury invariably led to more weight problems. But this time, for once, she decided to do it differently: she would forget about diets and regimes, she would just try to relax. She booked herself into an eco lodge in Costa Rica, turned off her phone, forgot about tennis and might have beens, did some yoga, took long walks, and for the first time in a decade found herself, to her surprise, wanting to eat fruit rather than "dreaded carbs".

When she got home she went through all of her photographs and clippings, relived every high and low of her life, and started to mourn not for her career but for her father. It was as if a light had come on. So deep had the idea of "no pain no gain" been ingrained in her that for a time the gentler regime she allowed herself in the weeks that followed seemed unnatural. She walked instead of running and "on those walks I slowly and sadly came to terms with my life. I lost my dad way too early and it was agonisingly awful. I missed him so much and I hated knowing that I could never again pick up the phone to tell him about my day".

Seles came to realise that food had been her way of deflecting that pain; the grief that had cruelly coincided with her traumatic loss of innocence on court. She had kept it all in, she believed, but now she could see it for what it was. It was too late for her to go back to playing - her ankles saw to that, but she did begin to find a way to do that most difficult thing for ex-champions - to find a way to live outside the lines of the court. Money was not a problem - she had earned nearly $15 million on court alone (though, of course, without the interruption of her career, she may well have doubled or tripled that figure), but a sense of purpose was. Seles needed to defeat what she saw as the "toughest opponent of her career - her weight - once and for all".

She kept walking. She started to be honest with herself about what she was eating. She stopped punishing herself for what she could not do. The walking put her back in touch with the sense of how her body had once been her ally, had done anything she wanted it to. Her father had always helped her find a way of beating any opponent, and now she could see a way of beating this one. She stopped worrying about the grand slams she had never won, and she started to be proud of those she had. The mystery about her eating was that there was no mystery.

"Once I became honest about what was really go on in my head and with my emotions, then I could see a way through it," she says. "My mistake was to think there was an easy fix, a miracle diet. If I could sort out my weight, then everything would be all right again. I had it the wrong way round. It was not about what I was eating, but about what was eating me." It wasn't easy - it has taken all of the five years of her retirement for Seles to feel like she can face the world, but one thing she has been been used to is playing the long game. And like any great champion, she could always find a way to win.